Conventional glass mirrors for automotive use are heavy (especially with the trend towards larger mirrors), tend to be difficult to form aspheric or complex shapes, and are prone to shatter in the case of accidents. The replacement of glass with a plastic substrate would offer the ability to mould complex shapes, to integrate retention/clipping features on the rear, to reduce weight, and to achieve a high level of impact resistance. However, plastic substrates tend to be soft, would be readily damaged by UV radiation, heat and water, and would have a propensity towards causing the reflective surface to craze.
In addition to the softness problems that would expect to be introduced by the use of a plastic substrate for mirrors, some reflective layers typically used for mirrors (of any type) that are formed from direct metal deposition (such as of aluminium or silver) also themselves tend to be soft. To minimise this problem with glass substrates, most reflective layers are conventionally applied to the rear surface. However, for reflective metals such as chromium, which are harder and more resistant to the elements by virtue of their chemical and mechanical properties, these metals are predominantly applied to the front surface of glass substrates.
However, if this were attempted with a plastic substrate, it would be expected that the reflective coating would exhibit crazing, which would be made worse in durability testing such as thermal cycling, and which would likely also fail other standard tests such as a salt spray test (being either DIN 50 021 or ASTM B117).
In the case of plastic mirrors, the reflective layer could be applied to the rear or front surface and a hard coating applied in order to provide environmental exposure protection of the plastic substrate and the reflective coating, whilst providing improved abrasion resistance. However, for an automotive application, a rear surface plastic mirror has not been regarded as suitable as it would be interrupted by any retention/clipping features on the rear of the mirror. Also, in the case of an applied hardcoating, interference bands would be present which are cosmetically unacceptable.
It would thus be desirable to manufacture a plastic mirror, suitable for automotive applications, which does not craze, which does not have interference fringes and which passes the normal automotive tests such as a salt spray test, thermal cycling and shock tests, and accelerated weathering tests.
The above discussion of background is included to explain the context of the present invention. It is not to be taken as an admission that any of the material referred to was published, known or part of the common general knowledge at the priority date of any one of the claims. Also, before turning to a general description of the invention, and for the sake of clarity, it should be appreciated that references throughout the specification to the “front” of a mirror are references to the side of the mirror upon which reflected images are ordinarily viewed by a user.